The notion of providing dipolar and tripolar antennae, at least in terms of providing two or three orthogonally aligned antennae, is known, particularly in the field of wireless data communication, such as is employed in wireless local area network (LAN) cards, Bluetooth®, wireless routers and the like. It has also been considered in the field of mobile cellular telecommunications where the provision of one or more secondary antennae might provide a more reliable, stable, improved and/or effective connection between the base station or signal broadcast mast and the mobile device. For instance, Proc. IEEE, vol. 92, Feb. 2004 (Paulraj et al.) demonstrates that multiple antenna designs are much desired in wireless applications, as they provide significant improvements in signal reliability and data rates.
Polar antennae arrangements are also known to be spatially more efficient in that they can be effectively miniaturized, and as such prove invaluable for mobile cellular devices where overall product size is of critical importance. (See IEEE Trans. Commun., vol. COM-20, Oct. 1972, Lee & Yeh).
A technique known as spatial multiplexing is also known wherein three separate data signals can be transmitted by transmitting/receiving three separate electromagnetic waves, each having one of the three possible linear polarizations of the electric field. It is to be mentioned that the polarisation axes are traditionally, but not necessarily, orthogonal. The ability to communicate three data streams simultaneously in this manner can increase the overall data throughput by a factor of 3 (Nature, vol. 409, Jan. 2001, Andrews, Mitra, and Carvalho).
U.S. Pat. No. 6,844,858 to Andrews, Mitra, and Gans further discloses that in a rich scattering environment, there are potentially six, and not merely two or three, independent polarization channels available for the transmission of data. However, this allegation is considered contentious because for free space propagating wave, the magnetic field component is a relativistic manifestation of the electric field component. Therefore there are only 3 available channels, ignoring inductive non-propagating effects. Notwithstanding this, the document discloses a simplified version of a tri-polar antenna which might be considered for use in the invention described. This is shown in FIG. 1 hereof, and the two dipole elements 100 and 105 and the loop element 110 which are said to provide the threefold polarization diversity, and the orthogonal arrangement of two of dipole elements 100 and 105 can clearly be seen. Importantly, this arrangement is described in the patent as being useful for transmitting and/or receiving over a subset of the notional six polarization channels, and specifically a 3 channel communication system is proposed using two mutually orthogonal electric field polarisations and a magnetic field polarised in the third orthogonal direction. As will be immediately understood by those skilled in the art, from FIG. 1 the two perpendicularly arranged dipoles transmit/receive the two mutually orthogonal polarisations of electric field, whereas the loop element transmits/receives the magnetic field polarised in the third orthogonal direction, i.e. predominantly about an axis coincident with the geometric centre of the loop.
One disadvantage of the antenna shown in FIG. 1 is that each of the three antenna are superposed. In the patent, the arrangement of the various elements is described as being substantially in a plane, but actually planar configuration is impossible due to superposition of each of the elements. In applications where space is at a premium, such arrangements may be precluded.
A more pervasive disadvantage of this arrangement is that the loop element does not provide a useful antenna for the reception or transmission of an electric field polarised in the third orthogonal direction. Indeed, the fundamental inventive realisation behind U.S. Pat. No. 6,844,858, and by which it is concluded that there are potentially six independent polarisation channels, is that for a suitable arrangement of antennae transmitting or receiving electromagnetic waves, there are 3 components of electric field, namely two transverse components, and a longitudinal component. The transverse components are capable of propagating from one antenna to another as a result of being disposed in a rich scattering environment, while the longitudinal component propagates directly through space.
As is known by those skilled in the art, electromagnetic waves are generally polarised in directions orthogonal to the propagation direction, and thus E and H components (from Maxwell's equations governing electromagnetic theory, E being the electric field vector and H being the magnetic field vector) are typically orthogonal to one another in the far-field—it is these physical factors which led the inventors to deduce that 6 independent channels (3 using E and 3 using H) may exist. However, it is known that E and H are only sufficiently de-coupled so as to be considered independent in the near-field (for example over the order of a few meters) as opposed to the far-field (for example between mobile telephones and their most proximate transmission/reception antenna, i.e. a few hundred meters or even a few km).
It is an object of the invention therefore to provide an antenna with multiple polarizations that is of generally planar configuration which is compatible with printed circuit board production techniques.
It is a further object of this invention to provide an antenna capable of transmitting/receiving electromagnetic radiation having three different electric field polarizations.